Finding Key Factors for Efficient Water and Methanol Activation at Metals, Oxides, MXenes, and Metal/Oxide Interfaces

Activating water and methanol is crucial in numerous catalytic, electrocatalytic, and photocatalytic reactions. Despite extensive research, the optimal active sites for water/methanol activation are yet to be unequivocally elucidated. Here, we combine transition-state searches and electronic charge analyses on various structurally different materials to identify two features of favorable O–H bond cleavage in H2O, CH3OH, and hydroxyl: (1) low barriers appear when the charge of H moieties remains approximately constant during the dissociation process, as observed on metal oxides, MXenes, and metal/oxide interfaces. Such favorable kinetics is closely related to adsorbate/substrate hydrogen bonding and is enhanced by nearly linear O–H–O angles and short O–H distances. (2) Fast dissociation is observed when the rotation of O–H bonds is facile, which is favored by weak adsorbate binding and effective orbital overlap. Interestingly, we find that the two features are energetically proportional. Finally, we find conspicuous differences between H2O/CH3OH and OH activation, which hints toward the use of carefully engineered interfaces.


INTRODUCTION
Water plays a crucial role in numerous catalytic reactions. It can either act as a reactant for surface reactions such as the water−gas shift and methane/methanol steam reforming or facilitate reactions as moisture in the reactant gases. 1−8 In addition, it is used as a solvent in countless inorganic and organic reactions and is also important in electrochemistry, fuel cells, and corrosion science and technology. 9,10 Furthermore, apart from being a commodity chemical, methanol has attracted great interest in recent years for hydrogen production via methanol steam reforming, the development of direct methanol fuel cells to be used in small portable devices, and the potential of CH 3 OH photocatalytic oxidation. 2,11−15 In view of their high thermodynamic stability, the activation of water or methanol is habitually a decisive part of catalytic pathways, often the rate-limiting step. 1,2,13,16−20 Although numerous studies have been devoted to identifying the active sites for water and methanol activation, they are still a matter of debate in view of the coexistence of numerous structural motifs at catalytic surfaces. For instance, some authors have suggested that oxide supports (e.g., TiO 2−x , CeO 2−x ) are responsible for water activation in the water−gas shift. 16,21−23 Others have shown that both metals and oxide supports at metal/oxide interfaces (e.g., Cu/FeO x , Ni/TiO 2−x , Au/TiO 2−x ) directly participate in water activation. 24−26 Moreover, some authors claim that metals (Cu) or metal cations (Pt δ+ , Au δ+ ) are the active sites for the water−gas shift. 27−29 In addition, discrepancies exist about methanol activation on CuZn alloy sites or Cu/ZnO interfaces at Cu/ ZnO catalysts during catalytic methanol steam reforming. 30−35 These conflicting views greatly hamper the design and implementation of improved catalysts and call for fundamental studies that outline the different interactions between H 2 O/ CH 3 OH and various structural motifs/sites. In this context, the challenge lies in identifying the common features of swift activation kinetics among structurally different materials.
In this study, we identify two such features among metals, oxides, MXenes, and metal/oxide interfaces combining the climbing-image nudged elastic band (CI-NEB) method 36 for the location of transition states (TSs) and the Bader charge analysis. 37 Specifically, Cu(111), Co(0001), Pt(111), rutile TiO 2 (110), and Ti 3 C 2 O 2 (0001), together with Pt/FeO and Cu/ZnO interfaces are used to model various structural motifs/sites in view of their superior performance in relevant applications, such as water−gas shift, methanol steam reforming, and CH 3 OH photocatalytic oxidation. 1,2,15 The first feature refers to the charge state of H moieties, as there are clear energetic differences between H-like transfer and proton-like transfer during H 2 O, OH, and CH 3

METHODS
Spin-unrestricted density functional theory (DFT) calculations were performed with the Vienna Ab initio Simulation Package (VASP). 38 The interaction between ionic cores and valence electrons was described by the projector-augmented wave (PAW) method, 39 and the Kohn−Sham valence electronic wavefunction was expanded using a plane-wave basis set with a kinetic energy cutoff of 400 eV. Exchange−correlation effects on the total energies were calculated within the generalized gradient approximation (GGA) using the Perdew−Burke− Ernzerhof (PBE) exchange−correlation functionals. 40 Section S9 in the Supporting Information (SI) shows that the effect of D3 dispersion corrections on the adsorption energies is mostly a constant downward shift with a minor effect on the trends. 41 The total energies were converged to within 10 −4 eV, and the forces on the atoms were converged to within 0.05 eV/Å. The lattice constants for bulk Cu (face-centered cubic (fcc)), Co (hexagonal close-packed (hcp)), Pt (fcc), TiO 2 (rutile), and the Ti 3 C 2 O 2 (MXene) were calculated to be 3. 64 Cu(111), Co(0001), and Pt(111) were modeled using fourlayer slabs with (3 × 3) surface unit cells (Figure 1a). The surface Brillouin zones were sampled with (4 × 4 × 1) Monkhorst−Pack k-point grid meshes. 44 The two topmost layers and the adsorbates were fully relaxed, and the remaining layers were fixed at the converged bulk positions. (2 × 1) Fourlayer and five-layer slabs were used to model TiO 2 (110) and Ti 3 C 2 O 2 (0001) surface; see Figure 1b,c. The two topmost layers of TiO 2 (110) and all of the layers of Ti 3 C 2 O 2 (0001) together with the adsorbates were relaxed. The Brillouin zones were sampled with (4 × 4 × 1) and (5 × 5 × 1) Monkhorst− Pack grids. 44 A one-layer graphite-like (3 × 3) ZnO(0001) ribbon, with an in-plane lattice of 3.30 Å, on a three-layer (4 × 8) Cu(111) slab was adopted to simulate the Cu/ZnO interface ( Figure 1d). The Brillouin zone was sampled with a (1 × 2 × 1) Monkhorst−Pack grid. The two bottommost Cu layers and the four leftmost ZnO columns were frozen, while the remaining atoms in the metal slab and the oxide together with the adsorbates were relaxed. The Pt/FeO interface was modeled by a (2√3 × 5) rectangular supercell, including a bilayer FeO ribbon with three columns of Fe atoms and two columns of O atoms on a three-layer Pt(111) slab, as shown in Figure 1e. A single k-point located at (0.25, −0.25, 0) was used to sample the surface Brillouin zone. The Pt layers and the three rightmost FeO columns were frozen, while the remaining atoms in the oxide were relaxed together with the adsorbates. The DFT + U approach was used to correct the on-site Coulomb repulsion of 3d electrons of Zn and Fe atoms in the Cu/ZnO and Pt/FeO interfaces, with U−J values of 4.7 and 3.0 eV, respectively. 45,46 A vacuum region of at least 15 Å sufficed to avoid interactions between periodically repeated slabs along the z-direction for all of the systems studied. More details about the models can be found in previous works. 47 −49 The adsorption energy (ΔE Ads ) was calculated using H 2 O, CH 3 OH, OH, and H 2 in the gas phase as reference states since they are reasonably well described within DFT. 50,51 A lower (more negative) ΔE Ads implies stronger binding, while a higher (more positive) ΔE Ads implies weaker binding. All transition states (TSs) were located by the CI-NEB method, 36 and saddle points were confirmed by vibrational frequency analysis. The relaxations stopped when the residual forces on each atom were smaller than 0.05 eV/Å. The elementary activation barrier (ΔE Act = E TS − E IS , where TS and IS stand for transition and initial states, respectively) and reaction energy (ΔH = E FS − E IS , where FS stands for final state) were calculated with respect to the co-adsorbed states of the species on the surfaces (for instance, ΔH H 2 O =E *H+*OH − E *H 2 O ). We decompose the overall activation energy into two parts, namely, a preconditioning barrier and a dissociation barrier: ΔE Act = ΔE 1 + ΔE 2 . We note that to univocally define the preconditioning state, the rotation of O−H bonds and their stretching need to be successive events. However, our CI-NEBs have no specific constraints along the reaction coordinate such that the end of a rotation coincides with the stretching of O−H bonds by no more than 0.09 Å for all of the molecules studied; see Δd 1 in Table S1. We evaluated the effect of such overlap between rotation and stretching on the energy of the preconditioning states ΔE 1 (see Tables S2 and S3 and more details in Section S1) and found that the small variation observed in O−H bond distances (<0.09 Å) from the initial states to the preconditioning states does not change the main conclusions of the present analysis ( Figure S1). Compared to Δd 1 , the variation in the dissociating O−H bond distance between transition states and preconditioning steps (Δd 2 ) is significantly larger, falling in the range of 0.04−0.71 Å (Table S1). We also define the rotation angle (∠ABC) of the dissociating O−H bond from the initial to the preconditioning step in Figure S2 and Table S4 to better describe the rotation of O−H bonds.
The Bader charge analysis was performed using a grid-based weight method 37 in which the expression for the fraction of space neighboring each grid point that flows to its neighbors is used as a weight for the discrete integration of functions over the Bader volume. In this context, a positive or negative charge means charge depletion or charge accumulation, respectively.  Table S5. Such atop sites have been experimentally identified at low coverage and low temperature (<20 K) by scanning tunneling microscopy on Pt(111) 52 and Cu(110), 53 and predicted with DFT calculations to be the most stable adsorption sites on a number of close-packed and open metal surfaces. 54,55 This is because the dipole moment of H 2 O molecules at these sites is aligned almost parallel to the surface plane, which favors the interaction of the 1b 1 molecular orbital of H 2 O with the surface bands. 56 Taking adsorbed H 2 O as the initial state (IS), we studied its dissociation on Cu(111), Co(0001), and Pt(111); see Table  S6 Figure S2 and Table S4. As a result, the water molecule is closer to the surface (generally by about 0.45 Å) with respect to the IS, and the O and H atoms differ in height above the surface. As shown in Table S1 and Figure 2a (111) (Table S4). CH 3 OH moves closer to the surface (generally by about 0.61 Å) during the surface diffusion and rotation, with a small change in the O−H bond lengths (0.02 Å), energies (0.37 eV), and Bader charges of the dissociating H (0.04 e − ); see Table S1 and Figure 2c,d. After that, the O−H distance increases appreciably (by 0.43 Å), and so does the energy (by 0.75 eV) until the TS is reached. At the TS, the O− H bond is broken with H now bound to Cu, and the Bader charge of the dissociating H is lowered by as much as 0.52 e − relative to the IS. Therefore, similar to H 2 O dissociation, CH 3 OH dissociation on metal surfaces is a H-like transfer process requiring significant charge transfer and energy expenses.
Moreover, the reaction coordinate of OH dissociation on Cu(111) is composed of a rotation of the O−H bond from an almost perpendicular configuration to one that is parallel to the surface plane ( Figure S2 and Table S4), followed by its elongation. As shown in Figure 2e, (Table S6 and Figures 2, S3, and S4) agree well with previous DFT studies. 13 (Figure 1b) binds H 2 O and CH 3 OH more strongly, with ΔE Ads of −0.90 and −0.99 eV, respectively (Table S5). These results can be rationalized considering the more significant electron donation from the lone-pair electrons of O (2p z ) in H 2 O and CH 3 OH to the empty 3d states on TiO 2 (110) vs 3d and 4s states on Cu(111). This is seen from the deep-lying orbital hybridization in the energy window between −9 and −3 eV in Figure S5a,b. We attribute this to Ti atoms at the bridge O vacancy of TiO 2 (110) having more empty states than metallic Cu to accept lone-pair electrons.
Extracting H from the most stable states of adsorbed H 2 O at the bridge O vacancy to yield two adjacent bridge OH moieties on TiO 2 (110), has a reaction barrier of 0.34 eV (Table S6), in agreement with previous DFT studies. 61 Figure  3a and Table S1. After that, the O−H bond is elongated to 1.21 Å, and the TS is subsequently reached upon a small energy cost of 0.22 eV. This is noticeably different compared to metals, which have a substantial energy increase (0.85 eV on Cu(111) in Figure 2a (Table S6 and Figure 3c). Again, the low barrier is linked to a proton-like transfer during the dissociation process, wherein no significant change in the charge of the H moiety is noticed (Figure 3d).
*OH dissociation at the bridge O vacancy on TiO 2 (110) has a considerably higher ΔE Act (1.26 eV) than H 2 O and CH 3 OH dissociation; see Table S6   3f), the high ΔE Act must stem from other reasons. The reaction coordinate proceeds through an initial O−H bond rotation to form a hydrogen bond with an adjacent bridge O site (Table  S4), with a large associated energy cost of 0.62 eV (Figure 3e). This is in contrast with H 2 O and CH 3  With respect to TiO 2 (110), the binding energies of H 2 O and CH 3 OH on Ti 3 C 2 O 2 (0001) are slightly weaker by no more than 0.10 eV, whereas the binding energies of dissociated species such as *OH and *CH 3 O are considerably stronger by 0.34−0.46 eV; see Table S5. Accordingly, not only are H 2 O and CH 3 OH dissociations more exothermic on Ti 3 C 2 O 2 (0001) than on TiO 2 (110), but ΔE Act also decreases by 0.21−0.30 eV on this MXene (Table S6 and Figure S6). However, the case is different for OH dissociation, which has ΔE Act = 1.40 eV on Ti 3 C 2 O 2 (0001), slightly higher than on TiO 2 (110) by 0.14 eV. Again, the facile dissociation of H 2 O and CH 3 OH is linked to a proton-like transfer process, whereas strong OH adsorption (−5.20 eV) is responsible for the unfavorable dissociation activity on Ti 3 C 2 O 2 (0001), as discussed above for TiO 2 (110). Finally, we note that both TiO 2 (110) and Ti 3 C 2 O 2 (0001) bind OH at the bridge or hollow sites with rather negative adsorption energies, which leads to high dissociation barriers. However, oxides binding OH on top sites may display weaker adsorption energies and, thus, more facile O−H bond rotation and faster dissociation kinetics.
To close this section, we note that, in agreement with our observations, Chandler et al. found through a combination of kinetics experiments, infrared spectroscopy experiments, and DFT calculations that H-like and proton-like transfers lead to dissimilar activities for H 2 dissociation on TiO 2 -supported Au catalysts. Specifically, the heterolytic H 2 dissociation, resulting in a formal hydride adsorbed on Au sites and a proton bound to the support to produce a TiOH group (proton-like transfer), has a lower barrier than the homolytic H 2 dissociation on Au sites (H-like transfer) by 0.46−0.57 eV. 62 3.3. H 2 O, CH 3 OH, and OH Activation on Cu/ZnO and Pt/FeO Interfaces. We now turn our attention to metal/ oxide interfaces, in particular Cu/ZnO and Pt/FeO. As shown in Figure 4, Figure S7), with a ΔE Ads that is 0.63 eV more negative than on Cu(111). These adsorption properties result in more favorable thermochemistry for H 2 O, OH, and CH 3 OH dissociation on Cu/ZnO, with ΔH values from −0.24 to −0.37 eV (Table S6).
As shown in Figure 4, H 2 O, OH, and CH 3 OH dissociation on Cu/ZnO proceed through proton-like transfer processes, with a remarkably low ΔE Act of 0.01−0.16 eV (Table S6). In particular, OH dissociation has ΔE Act = 0.16 eV, which is substantially lower than those of metal and oxide surfaces (in the range of 0.95−1.60 eV, see Table S6). H 2 O and CH 3 OH form strong hydrogen bonds with interfacial O atoms upon adsorption at Cu/ZnO (Figure 4a,c), which avoid O−H bond rotation over wide angles ( Figure S2 and Table S4) (Figure 4).
The Fe-terminated Pt/FeO interface has a strong oxygen affinity, evinced by its OH and CH 3 O binding energies (Table  S5). In fact, they are more negative than those of Pt(111) by 0.77 and 0.68 eV. However, atomic O binds at the Pt/FeO interface more weakly than on Pt(111) by 0.59 eV. This is because atomic O is only coordinated to an Fe atom at the interface, while three Pt atoms are available on Pt(111) ( Figure  S7Ca,Ga). In addition, H 2 O, CH 3 OH, and H at the interface have comparable ΔE Ads to Pt(111). These results indicate that H 2 O and CH 3 OH dissociation are thermodynamically more favorable on Pt/FeO interfaces than on Pt(111) (Table S6), whereas the case is markedly different for OH dissociation.
As shown in Table S6 and Figure S8a Table S7 Table S7).

Systematic Trends in the Scission of O−H Bonds.
Beyond the case-by-case analysis in Figures 2−4, S3, S4, S6, and S8, it is possible to extract overall trends from the data in this study. ΔE Act can be split into two parts, namely, a preconditioning barrier and a dissociation barrier (hereon denoted as ΔE 1 and ΔE 2 , respectively; see Table S7; see also Sections 2 and S1 for the determination of preconditioning states). ΔE 1 is mostly related to the rotation of the O−H bonds, whereas ΔE 2 corresponds to their actual cleavage. Figure 5 shows that ΔE Act , ΔE 1 , and ΔE 2 are approximately correlated in a linear manner, implying that the costs of rotating and cleaving O−H bonds are adsorbate-and materials-specific yet proportional. All in all, lower ΔE Act is consistently observed on oxidized materials and metal/oxide interfaces compared to metals. In addition, it is generally easier to cleave water and methanol than OH, except for Cu/ZnO interfaces, which cleave the three adsorbates with equally low barriers.
Furthermore, the inset in Figure 5b (blue line) shows that Brønsted−Evans−Polanyi (BEP) relations 65−67 hold for OH dissociation on the materials under study. These relations connect a thermodynamic variable (ΔH) easy to calculate using DFT with a kinetic variable (ΔE Act ) obtained through complicated transition-state searches. Although BEP relations are not observed for the activation of water and methanol, the inset in Figure 5 shows that their adsorption energies are proportional and so are the activation energies of their dissociation. Besides, Figures S9−S11 show that the adsorption energies of H 2 O and CH 3 OH, the total change in Bader charge (ΔBC) from the initial to the transition state, ΔBC 2 , and the geometric mean of the Bader charges between the initial and transition states (denoted as G(BC IS , BC TS )) are well correlated with ΔE Act . In addition, since the Bader charges at the transition states might be difficult to assess, we found a correlation between the mean Bader charges of initial and transition states and the Bader charges of the final states, as shown in Figure S12. In sum, Figures 5 and S9−S12 suggest that, in spite of the wide diversity of the materials under study, there are energetic and electronic descriptors that might be used to devise high-throughput routines to search for efficient catalysts to cleave methanol, water, and/or OH. Interestingly, a good catalyst for H 2 O activation is most certainly good for methanol activation, and vice versa. However, only in the presence of strong hydrogen bonding at interfaces might OH be inexpensively cleaved.

CONCLUSIONS
Knowledge of the underlying factors determining water and methanol activation is necessary for the design of enhanced catalysts for numerous reactions in catalysis. Finding such factors is usually complicated in view of the heterogeneity of the materials used to catalyze those processes and because of the great computational expenses associated with the assessment of kinetic barriers at surfaces and interfaces.
Here, through an interplay of CI-NEB transition-state searches and Bader charge analysis, we identified the key roles of proton-like transfer and O−H bond rotation in H 2 O, OH, and CH 3 OH activation on metals, oxides, MXenes, and metal/oxide interfaces. We provided a unifying framework for understanding the activation of O−H bonds, which allowed us to identify the active sites where it is more favorable, namely, at oxidized materials and metal/oxide interfaces, preferably offering strong hydrogen bonds. At those sites, O−H bond scission is accompanied by a proton-like transfer of H moieties and easy rotation of O−H bonds. Furthermore, we observed that an active material for cleaving water is likely suitable for methanol activation, but this need not be the case for OH dissociation.
The energetic proportionality between easy rotation and efficient O−H bond cleavage together with BEP and similar relations shown here might be used for the high-throughput in silico design of improved catalysts for reactions of industrial and technological interest in heterogeneous catalysis (e.g., methane/methanol steam reforming), electrochemistry (e.g., water splitting), and photocatalysis (e.g., methanol oxidation), where water, hydroxyl, and methanol are often present as reactants, intermediates, or products. In particular, the conspicuous differences between H 2 O/CH 3 OH and OH activation hint toward the use of carefully engineered multisite catalytic interfaces with controllable hydrogen bonding.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.1c03405. Determination of preconditioning states; energetic and geometric data for adsorbates and elementary reactions; energies and Bader charges of dissociating H moieties along the reaction coordinates; projected density of states; correlation between activation energies, adsorption energies and Bader charges for H 2 O and CH 3 OH cleavage; and converged Cartesian coordinates and electronic energies (PDF) Technology [KCYKYQD2017017]. The grants RTI2018-095460-B-I00, RYC-2015-18996, and MDM-2017-0767 were funded by MCIN/AEI/10.13039/501100011033 and by the "European Union". This work was also partly supported by Generalitat de Catalunya via the grant 2017SGR13.